Bleaching of cellulosic matter



Patented Feb. 8, 1949 BLEACHING OF CELLULOSIC MATTER Rudolf Bloch,Jerusalem, Kurt Goldschmid, Tel Aviv, and Isaac Schnerb and PaulGoldschmidt,

Jerusalem, Palestine No Drawing. Application December 17, 1946, Se-

rial No. 724,696. In Great Britain January 1,

Claims.

This invent-ion relates to the bleaching and refining of cellulosefibres, and materials containing cellulose fibres, such as textilefibres, yarns of fabrics, paper, cotton wool, wood pulp, straw or thelike. The invention is more particularly concerned with the treatmentcarried out with aqueous solutions of alkali or alkaline earth,hypochlorites. The invention also applies to the treatment of crudecellulosic materials with elementary chlorine or oxidizing chlorinecompounds with a view to producing a fibre pulp.

Hypochlorite bleaching as hitherto practised has several drawbacks, Theoperation is slow unless the Vet has. a very high initial concentrationof availablechlorine, or the temperature is raise'd,'but this wouldinjure the fibre.

A further considerable decrease of the speed of the operation occurstowards the end, in consequence of which the available chlorine can inpractice not be utilized exhaustively but the operation has to be brokenoil when about one third 'to 9, practically hampers the action of thechlorine. Thissensibility of the operation to the pH'value of the vat isvery inconvenient for the pH of the vat tends to decrease during theoperation which would make it desirable to start the operation with arather high pH value. Thus, a careful observation and regulation of thereaction is required during the operation.

Many attempts have been made at improving thehypo'chlorite bleaching inorder to obtain acceleration of the process, complete whiteness withoutsecond bleaching, and full utilisation of the available chlorine, but nosatisfactory solution of these problems has so far been found.

This invention is based on the discovery that the drawbacks referred toabove can be overcome if the hypochlorite treatment is carried out inthe presence of bromine ion.

.By the presence 'of bromide; the available chlorine is made moreactive, which entails two main consequences. Firstly, the. bleachingop-.

eration is markedly accelerated, that is, the consumption of availablechlorine, proceeds at a quicker pace. Secondly, rather lowconcentrations can be made to perform the bleaching in a short time. Ofthis second feature, use can be made either thereby that the availablechlorine of a vat is almost completely exhausted in one bath, or a spentliquor containing still some available chlorine is used for a subsequentbath until complete exhaustion of the chlorine, whereafter a freshamount of hypochlorite, may be, added.

Both features aforesaid, that is, acceleration of the process andexhaustive utilization of available chlorine, make it possible to. startthe operation at a lower concentration of available chlorine than isused heretofore, with the result that the danger of affecting thetensile strength of the fibres is diminished.

The usually desired high degree of whiteness can be obtained in onesingle operation. In this manner, and in view of the accelerationaforesaid, the time spent for the bleaching operation proper isdecreased to almost half of what it has been hitherto.

The operation is less sensitive to the alkalinity of the vat, and aninitial pH value of about 10 to 11 may be used without inconvenience.This facilitates the operation, for on the one hand there is no need fora very careful removal of the alkali adhering to the fibres when theycome from the preparatory treatment in the kier, on the other hand thevat may have so high an initial alkalinity that in spite of the gradualdrop of the pH, the reaction of the vat remains "suificiently alkaline.This greater ins-ensibility of the operation to the alkalinity makes itpossible to dispense with the careful observation and regulation of thereaction.

The bromide contents of the vat may be of the order of 0.5% of bromineion calculated to the weight of the goods to be bleached, or about 0.25g. of bromine ion per litre of the bleaching vat.

The addition of bromide to the vat may be effected at the beginning ofthe bleaching operation. In this case, much of the available chlorine isconsumed at once, about during the first quarter of an hour, and furtheramounts of hypochlorite are suitably then added to the vat in order thatthe initial concentration of available chlorine be substantiallyrestored. As compared with ordinary hypochlorite bleaching the initialconcentration of available chlorine may be lower, about 0.7 g. perlitre.

On the other hand. the bleachin operation may be started withhypochlorite alone and the bromide may be added later on, for example,after one half up to two thirds of the initial amount of availablechlorine have been used up.

According to a modification of the invention aforesaid the addition ofthe bromide to the bleaching liquor is spread over a great part or thewhole of the time required for the bleaching.

The term addition comprises the introduo- The total amount of bromine inthe bleaching liquor should not exceed about one third of the weight ofavailable chlorine, or about one sixth of a mole of bromine for eachmole of available chlorine.

The addition of the bromine may be effected in a continuous flow orintermittently in small portions, and it may be started either at thebeginning of the bleaching or after some bleaching with hypochlorinealone.

It has further been observed that while the bleaching of cellulose withhypochlorite alone is preferably and usually carried out at a pH above9, the adjustment of the hydrogen ion concentration of abromide-containing hypochlorite liquor to a pH value of 7 to 8.2 entailstwo unexpected major advantages: the speed of the 1 bleaching reactionis greatly increased, and the quality of the product, expressed in termsof fluidity (determined in accordance with the pre* scriptions ofClibbens and Geake, Shirley Inst.

Memoirs, 1927, No. 6, page 117) is much improved.

The invention, therefore, also comprises the adjustment of the pH valueof the bromide-containing bleaching liquor to Within the range stated.

It has been observed that by thus spreading the addition of bromide overa greater part of the bleaching time the same advantages, as comparedwith hypochlorite bleaching without bromide, can be obtained as in theembodiments of the invention described hereinbefore, but with aconsiderably smaller amount of bromide, This offers the additionaladvantages that the process becomes cheaper and the fibre is virtuallynot attacked by the small amount of hypobromite present in the liquor atany time.

Like the aforesaid embodiments of the invention, this embodiment also isapplicable to cellulosic material of various description. It has provedparticularly valuable for the bleaching of wood pulp. In addition thisembodiment is very suitable for bleaching textile fibres, yarns orfabrics in a continuous operation by running them through a vat, orpreferably through several vats, or several compartments of one and thesame vat, in, succession, which they leave ready for the usualafter-treatment. This can be done owing to the acceleration of thebleaching by the addi-- tion of bromide to the bleaching liquor andowing, moreover, to the surprising fact established by the inventorsthat the combined action of hypochlorite and bromide on the cellulosicmaterial allows the kiering treatment of the latter to be shortened.

The invention is illustrated by the following examples to which it isnot limited.

- Example 1 100 kg. of cotton cloth are bleached in a vat of 2000 litrescontained in a bleaching machine provided with usual winches over whichthe cloth runs as an endless band, a section of the latter hanging downinto the vat. To the vat, so much of a concentrated aqueous solution ofsodium hypochlorite is added that the concentration of availablechlorine in the vat is about 0.7 to 0.8 g. per litre. Moreover, 750 g.of potassium bromide are dissolved in the vat before the bleachingoperation is started. During the first 15 minutes of the bleachingoperation some more sodium hypochlorite solution, in the total amount of800 g. of available chlorine, is gradually added. This amountcorresponds to the consumption of chlorine during the first fixteenminutes so that the initial concentration is substantially restored.During the subsequent hour the chlorine concentration drops to about 0.4to 0.5 g. chlorine per litre, which marks the end of the operation.

The cloth is then withdrawn from the bleaching vat, after-treated in theusualmanner by washing in bisulphite and in soap solutions and rinsingin water, and is obtained completely white without requiringafter-bleaching.

If the same cloth were to be bleached in the usual manner withhypochlorite alone, the initial chlorine concentration would have to beof the order of 1.0 to 1.5 per litre, the operation would require atleast about two hours, and the end concentration of chlorine would be ofabout 0.5 to 0.9 g. of chlorine per litre. After the usualaftertreatment, an after-bleaching operation would be necessary sincethe goods are not obtained completely white after the first bleaching.

Example 2 With the same cloth as in Examplev 1, the bleaching operationis started with 2000 litres of a vat containing 1 to 1.2 g. of availablechlorine per litre. After about 30 to 45 minutes the chlorine contentshave dropped to about 0.7 to 0.8 g. per litre. At this stage, 750 g. ofpotassium bromide are dissolved in the vat, The bleaching operationtakes altogether about to 105 min utes.

Example 3 kg. of cotton cloth are bleached with the aid of a jigger in avat of 200 litres containing sodium hypochlorite in an amountcorresponding to a concentration of available chlorine of about 5 to 6g. per litre. After about half an hour the concentration of chlorine hasdropped to about one third its initial value, and 600 g. of potassiumbromide are then added to the vat. Complete whiteness is reached afterabout 1% hours from the beginning of the operation, and the availablechlorine is then practically exhausted.

Example 4 The bleaching operation is carried out as in Example 1, theonly difference being that instead of potassium bromide, a concentratedaqueous solution of sodium hypobromite-bromide prepared by the additionof bromine water containing 500 g. of bromine to a concentrated aqueoussolution of about 250 g. of N aOH, is added to the vat.

Example 5 The vat is prepared thereby that into a dilute aqueoussolution of sodium hydroxide containing about 4750 g. of NaOH, 4000 g.of gaseous chlorine and bromine water containing 500 g. tit-bromine aregradually introduced whereafter water is added up to a total volume of2000 liters. The bleaching operationis then carried out as in EX.- ample1.

I Example 6 In the manner known from the preparation of bleaching powder(chloride of lime), a bleaching mixture is prepared by the introductionof 4000 g. of elementary chlorine and bromine water containing 500 g. ofbromine into an aqueous suspensionof 6000' g. of calcium hydroxide, andthis mixture is diluted with so much water that a clear solution of atotal volume of 2000 liters is obtained which is used as vat in ableaching operation carried out as described in Example'i.

Example I Dry commercial wood-pulp is mixed with somuch water that a wetpulpy mass is formed. To this, a concentrated aqueous solution of sodiumhypochlorite is added in the amount necessary for obtaining the desireddegree of bleaching, which is previously determined with a small portionof-the pulp. As arule, an amount of from 6 to 12 g. of availablechlorine is required for each 100 g. of dry matter of the pulp, To this,potassium bromide is added in aqueous solution,

the amount of Br being 3 g. for each 100 g. of dry matter of the pulp.The vat is then allowed to stand for some-hoursuntil the desired degreeof bleaching. has been reached.

- Example I 2000 li res of wood-pulp having a dry-matter rat o of'5% (i.e. 5 kgs. of dry matter in each 100 litres of pulp) are added to with280 litres of an aqueous solution of bleaching powder (chloride of lime)containing a total amount of 7 kgs.

of available chlorine. i. e. 7% by weight of dry cellulosic matter. Thetemperatureof the liquor is ke t at; about 35 C. After one hour, whenabout 60 to 70% oi. the available chlorine have been used up. 300 litresof an aqueous solution of potassium bromide of 0.5% strength by weightare allowed to flow in during about one further hour. The total amountof'bromine ion thereby introduced is about 1% by weight of the drycellulosic matter. While the potassium bromide solution flows in, theliquor is stirred. When the addition or bromide begins, the hydrogen ionconcentration of the liquor is initially brought to pI-I=7.5 by theaddition ofacid, e. g. hydrochloric or formic acid, and during thefurther operation the hydrogen ion concentration is kept underobservation, and whenever necessary, adjusted by the addition of alkalihydroxide or milk of lime so as to remain more or less constant at aboutpl-I=7.5. At the end of about two hours from the beginning of thebleaching, the available chlorine has been fully consumed. The liquor isthen diluted with about one hundred times its volume of water,neutralized with sillphuric acid, and filtered, and the residue iswashed with water.

Example 9 bromine thus introduced being about 0.75% by weight of the drycellulosicmatter,

Example 10' .j

2000 litres of wood-pulp with a dry-matter ratio of 5% are added to with280 litres of. an aqueous solution of chloride of lime containingaltogether 7 kgs. of available chlorine. The liquor is kept at 45 C. Atonce with the addition of the chloride of lime, one starts with. theaddition to the vat of 600 litres of an aqueous solution of potassiumbromide of 0.5% strength by weight, which is spread over 45 minutes. Thetotal amount of bromine thus introduced is 2 kgs, or 2% of the drycellulosic matter. The whole of the available chlorine has been consumedafter about minutes, when the liquor is further treated as described inExample 1.

Example 11 v To a wood-pulp-hypochlorite mixture composed as describedin Example 1 and kept at about 25 0., a total amount of 3'75 litres ofan aqueous potassium bromide of 0.5% strength by weight is added, theaddition being started about one hour after the beginning of thebleaching and being spread over minutes. The total amount of brominethus introduced corresponds to 1.25% of the dry cellulosic matter. Thecon sumption of available chlorine has been completed after about 250minutes from the beginning of the bleaching, whereafter the mixture isWorked up as described'in Example 1.

posed as described in Example 1 and kept at about 45 0., 165 litres ofan aqueous solution of potassium bromide of 05% by weight are graduallyadded during minutes, starting with the beginning of the bleachingoperation, altogether 550 grs. of bromine, or 0.55% of the Weight of.thecellulosic dry matter. At the end of this time, the whole of theavailable chic-- rine has been consumed. The mixture is then furthertreated as described in Example 1.

Example 13 A web of cotton cloth is continuously run through a vatcontaining a. boiling solution of caustic soda with a content of 15 grs.of NaOH per litre, the rate of advance of the web through the bath beingsuch that each part thereofro mains in contact with the liquor for about20 minutes. The web then passes through water for washing and thereafterthrough a succession of bleaching vats containing hypochlorite liquorwith 0.4 gr. of available chlorine per litre and kept at a pH of7.0-8.0. To each vat an aqueous potassium bromide solution is suppliedcontinuously or in small portions at such a rate that the bromideconcentration in the bath is kept at about 0.2 gr. of bromine per litre.Makeup hypochlorite solution is also added to the vatscontinually or insmall portions so as' to keep the contents of available chlorine in thevats substantially constant. The make-up hypochlorite may be addedeither in mixture with washing bath,thereafter through another bath ofboiling caustic soda (containing 10 grs. of NaOH per litre), where eachpart thereof is again kept for about 20 minutes, then through anotherseries of bleaching vats containing a hypochlorite solution(withoutbromide) kept at 'a-con-' centration of 0.2 gr. of availablechlorine 'pe'r' 7 litre and a pH of about 9, to which each part of theweb is again exposed for about 20 minutes.

The web is then subjected to a usual aftertreatment.

Example 14 Kraft pulp is treated with hypochlor' e solution containingper cent, chlorine calculated on the pulp to be bleached, to which 0.2to 0.5 per cent. of bromine in the form of bromide calculated on the drypulp is added in three portions in the course of 30 minutes.

After 50 to 70 minutes the free chlorine is exhausted, then the pulp iswashed with a 2 per cent. soduim hydroxide solution calculated on thepulp at temperatures from the normal up to 100, filtered, washed, andagain bleached with 2.5 per cent. free chlorine, calculated on the drypulp, with the addition of a small amount of bromide. This stage takesanother 50 to 120 minutes. The Kraft pulp treated in this way is of asatisfactory full white. The strength of the pulp is not impaired andthe copper figure is low.

It is, of course, possible to have more chlorine used in the secondstage, and less in the first stage according to the quality of the pulp.

Example 15 Strong sulphite pulp is first chlorinated with chlorine water(the total amount of chlorine'used being 3.5% of the weight of the drypulp), and washed with an aqueous sodium hydroxide solution (2% byweight of NaOH calculated ondry pulp). Then the pulp is mixed with'waterto form a pulpy mass having a dry-matter ratio of 5%. Then so much of anaqueous solution of bleaching powder (chloride of lime) is added thataltogether 0.7% of chlorine (calculated on the dry matter of the pulp)is introduced. The temperature is kept at 37 C., and the pH at 9. .After45 minutes stirring, about 55% of the available chlorine have been usedup, and the pH is changed to 7.0-7.5 by the addition of somehydrochloric acid. Then an amount of an aqueous solution of potassiumbromide corresponding to 0.5% Br of the dry matter of the pulp isallowed to flow in during half an hour. After 90 minutes from thebeginning of the bleaching, 80% of the available chlorine have beenconsumed. The pulp is then washed with water and treated during 30minutes at ordinary temperature with a solution of sodium hydroxide (2%NaOH calculated on the dry matter of the pump). After washing with waterthe pulp is after-bleached with 0.1% C1 (in the form of a solution ofbleaching powder) at a temperature of 37 and pH of 9. After 30 minutesthe pulp is after-treated with S02. The fluidity of the bleached pulp is13.3.

Bleaching of the pulp with hypochloride alone, without adding bromide,to the same whiteness and with the same consumption of availablechlorine, takes five hours and the bleached pulp has a fluidity of 16.0.

Example 16 Easy bleaching sulphite-pulp with a dry-matter content of 5%,is treated with 6.5% Cl, calculated on the dry matter, in the form of asolution of bleaching powder, at a temperature of 37 C. and a pH ofabout 9. After one hours stirring 66% of the available chlorinehave'been consumed. Then the pH is changed to 7.0-7.5 by theaddition ofsome hydrochloricacid. A solution of potassium bromide, corresponding to0.5% B1 tit calculated on the dry matter of the pulp, is allowed to flowin during 30 minutes. After two hours from the beginning of thebleaching, of the available chlorine have been consumed. The pulp isthen washed and after-treated with S02. The fluidity of the bleachedpulp is 16.7.

Bleaching of the same pulp to the same whiteness with the sameconsumption of available chlorine with hypochlorite alone, withoutadding bromide, takes four hours and the bleached pulp has a fluidity of22.2.

This application is a continuation-in-part of copending application,Serial No. 610,676, filed August 13, 1945, now abandoned.

We claim:

1. Process oi bleaching cellulosic material which comprises treating itwith an aqueous hypochlorite liquor having a pH of at least 7 andcontaining water -soluble bromide in an equivalent amount smaller thanthe available chlorine content of the liquor.

2. Process of bleaching cellulosic material which comprises treating itwith an aqueous hypochlorite liquor having a, pH of at least 7 andcontaining water-soluble bromide initially added thereto in anequivalent amount smaller than the available chlorine content of saidliquor.

3. Process of bleaching cellulosic material which comprises treating itwith an aqueous hypochlorite liquor having a pH of at least 7 andcontaining water-soluble bromide added thereto in the course of thebleaching operation in an equivalent amount smaller than the availablechlorine content of said liquor at the time of the beginning of suchaddition.

, Process of bleaching cellulosic material which comprises treating itwith an aqueous hypochlorite liquor having a pH of at least 7 and addingto said liquor in the course of the bleaching operation and before theexhaustion of the available chlorine of the hypochlorite, water-solublebromide in an equivalent amount smaller than the content of availablechlorine of said liquor at the time of the beginning of such addition,said addition of bromide being spread over at least a substantial partof the total bleaching time.

5. Process of bleaching cellulosic material which comprises subjectingit to an initial bleaching with an alkaline reacting aqueoushypochlorite liquor, then, and before exhaustion of the availablechlorine of the hypochlorite, adding to said liquor water-solublebromide in an equivalent amount smaller than th content of availablechlorine of said liquor at the time of such addition, while adjustingthe pH of the liquor to a value between 7 and 8.2, and continuing thebleaching operation after such addition.

6. Process of bleaching cellulosic material which comprises subjectingit to an initial bleaching with aqueous hypochlorite liquor at a pH ofabout 9, then, and before exhaustion of the available chlorine of thehypochlorite, changing the pH to a value between 7 and 8.2, adding tosaid liquor water-soluble bromide in an equivalent amount smaller thanthe content of available chlorine of said liquor at the time of thebeginning of such addition, and continuing the bleaching operation, theaddition of the bromide being spread over at least part of suchcontinued bleaching while the pH value of the bleaching liquor ismaintained between 7 and 8.2.

'7. Process of bleaching cellulosic material which comprises treating itwith an aqueous hypochlorite liquor having a pH of at least 7 andcontaining a water-soluble bromide-hypobromite mixture with anequivalent bromine content smaller than the available chlorine contentof said liquor.

8 Process of bleaching cellulosic material which comprises treating itwith an aqueous hypochlorite liquor having a pH of at least '7, andadding to said liquor in the course of the bleaching operation andbefore exhaustion of the available chlorine of the hypochlorite, atleast one alkaline reacting compound selected from the group consistingof alkali metal hydroxides and carbonates, and elementary bromine in anequivalent amount smaller than the available chlorine content of saidliquor at the time of the beginning of the addition of bromine to saidliquor.

9. Process of bleaching cellulosic material which comprises subjectingit to an initial bleaching with aqueous hypochlorite liquor at a pH ofabout 9, then, and before exhaustion of the available chlorine of thehypochlorite, changing the pH to a value between 7 and 8.2, adding tosaid liquor at least one alkaline reacting substance selected from thegroup consisting of alkali metal hydroxides and carbonates, andelementary bromine in an equivalent amount smaller than the availablechlorine content of said liquor at the timeof beginning the addition ofbromine, and continuing the bleaching operation, the addition of saidalkaline reacting substance and bromine being spread over at least partof such continued 10 bleaching while the pH value of the bleachingliquor is maintained between 7 and 8.2.

10. Process of treating cellulosic material in a continuous processcomprising kiering and bleaching said material by running it through asuccession of baths, bleaching it in at least one of said baths with anaqueous hypochlorite liquor having a pH of at least '7 and containingbromide in an equivalent amount smaller than the available chlorinecontent of said liquor, and maintaining the contents of availablechlorine and bromine in said liquor substantially constant by thecontinued addition thereto of hypochlorite and bromide.

RUDOLF BLOCH. KURT GOLDSCHMID. ISAAC SCHNERB. PAUL GOLDSCHMIDT.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 125,658 Campbell Apr. 16, 18721,522,560 Weber Jan. 13, 1925 1,843,465 Traquair Feb. 2, 1932 1,883,193Wells Oct. 18, 1932 2,001,268 Rue May 14, 1935 2,265,033 Fryer Dec. 2,1941

